With the development of genetic engineering techniques, it is now possible to transfer genes from a variety of organism into the genome of a large number of different plant species. This process has many advantages over plant breeding techniques, as genes may now be transferred from one plant species to another plant species, rather than simply from a plant to the same, or different, but closely related, species.
Sucrose is one of the primary end products of photosynthesis in higher plants. It is also the major carbohydrate transported to sucrose accumulating, or carbon sink, tissues for plant growth and development (Pate, (1976) In: Transport and Transfer Processes in Plants, pp. 253-289, Wardlaw, J. F., Passioura, J. B., eds. Academic Press, N.Y.). Plant regions, such as leaf tissue, where sucrose is synthesized are commonly referred to as sucrose source tissue. Plant storage organs, such as roots or tubers, and fruits are examples of sink tissues.
Sucrose phosphate synthase (SPS) cDNA sequence, SPS constructs and transgenic SPS tomato lines are described in copending application, Ser. No. 08/175,471. This information is also published in Worrell et al. (The Plant Cell (1991) 3:1121-1130), incorporated herein by reference. In particular, the copending application describes a maize SPS under the control of the Rubisco small subunit promoter (SSU promoter) from tobacco, providing preferential expression in leaf tissue. Maximum activity of SPS is shown to be significantly increased in leaves of tomato plants expressing the maize SPS, and the absolute levels of starch and sucrose in the leaves are altered in the predicted manner.